How Long Can You Run a Generator When the Power Goes Out?

Updated: Jul. 24, 2023

When relying on a generator to get through a long-term power outage, you'll want to know how long you can run it before the lights go off.

Once you’ve enjoyed the benefits of a high-quality power generator, it’s hard to live without one. Where I live, in Northern Ontario, Canada, our electrical system seems to be less reliable than it once was, with at least a handful of power failures each year becoming the norm. Luckily, our portable gas-powered generator allows us to keep the lights and most of the appliances running.

Generators are useful for short term power outages, and essential for long term grid failure, but having one is just the first step. Once your generator is fueled, hooked up and running, the most important question to consider is how long it can run continuously. Answering the question, “how long can you run a generator,” isn’t easy because there are many factors involved. Here we’ll explain these variables and give you the clearest picture possible of how long you can expect your machine to keep your lights on.

Gasoline- and Diesel-Powered Portable Generators

Portable generators are small- to medium-size units putting out anywhere from 1,000 to 12,000 watts of electricity. Designed to be moved around to wherever they’re needed with relative ease, runtime depends on fuel tank size, fuel type and electrical demand. It also depends on whether your machine is an inverter or non-inverter model. Portable generators can run on gasoline, diesel or propane.

Regardless of fuel type, inverter generators have the ability to automatically throttle down when less electricity is demanded, then rev back up again when more power is required. This allows inverter generators (these are the best inverter generators for powering your RV and for providing electricity for your home) to have much better fuel economy than non-inverter models, which use governors (an attachment for automatic control of speed) to maintain a steady engine RPM regardless of electrical demand. All of these factors make it impossible to nail down an exact amount of time your portable generator can run, but you can expect to get six to 10 hours out of most gasoline- and diesel-burning non-inverter portable models, and 10 to 16 hours out of most inverters. A few machines have extra-large fuel tanks, so they can run for 18 hours or more. Keep in mind that inverters are also quieter and safer for sensitive electronics.

Propane-Powered Portable Generators

Working similarly to their gas- and diesel-powered counterparts, propane-burning portable generators are limited in their runtime by the size of the propane tank hooked up to them, along with electrical demand. A small 20-pound tank might get you five to eight hours. A 50-pound tank will last twice as long or more. You can also hook multiple tanks together with a multi-way valve or manifold system, allowing even more run time (150 hours or more.) At this point, the need to change oil every 100 hours or so (as most portable generator manufacturers recommend) will become your limiting factor.

Natural Gas-Powered Standby Generators

Physically larger and more electrically productive than portable models, standby generators are designed to be wired directly into your home’s electrical system, switching on automatically when there’s a power failure. Ranging from 7,000 to 15,000 or more watts of output, these units are intended to keep all or most of your electrical appliances fully operational during an outage. Standby generators use one of two fuel sources: propane or natural gas.

Natural gas units can keep running indefinitely, because their fuel will almost certainly outlast the outage. The limiting factor is oil. Most standby generator manufacturers recommend an oil change and other maintenance at least every two to three weeks of runtime, so in the event of an extended power failure, you’ll need to shut down your machine every few days to drain the dirty oil and replace it with fresh oil and a new filter.

Propane-Powered Standby Generators

Propane powered standby generators have the same oil changing requirement as natural gas models, with the additional factor of a limited fuel source. Your generator can only keep running for as long as your supply of tanked propane holds out, which depends completely on the size of your tank and your electrical demands. A large, 500-gallon propane tank will probably keep you going for four to six days of continuous runtime.

Solar-Powered Generators

Solar-powered generators aren’t like the generators mentioned above. They don’t involve combustion or fuel, but rather solar panels, a battery or bank of batteries and an electrical inverter. Unless you have your home fully outfitted with a huge amount of solar panels and an exceptionally beefy inverter, you probably won’t be able to use much power for very long on solar power alone. Most portable solar generators capture enough energy to power a fridge continuously (or as long as the sunshine holds out), but not much else at the same time. Expect to use your portable solar generator for only the most basic needs, and only some of the time. The upside is that these units never need refueling.